1. Field of the Invention
The present invention relates to a pneumatic tire having an improved tread pattern to reduce pattern noise. More particularly, the present invention relates to an improvement in a variable pitching method.
2. Description of the Related Art
In general, tires are provided in the tread portion with circumferentially spaced tread elements constituting a tread pattern such as blocks for good road grip. During running, such tread elements contact with the road surface one after another, and a noisy sound called pattern noise is generated.
In order to reduce such pattern noise, the so called variable pitching methods have been proposed. In those methods, pattern noise is modulated into a wide frequency range so that the pattern noise turns to the so called white noise. For example, in laid-open Japanese patent application Nos. JP-A-8-108711, JP-A-8-113012 and JP-A-8-113013, such a method is disclosed.
In the variable pitching methods, by increasing the number of different pitches, it becomes easy to turn pattern noise to white noise. However, when the pitch number is large, the rigidity of the tread elements is liable to become uneven and uneven wear increases.